The present invention relates to pigmented fibers of polypropylene (PP) and copolymers of propylene with a minor amount of a copolymerizable alpha-olefin monomer such as ethylene (P-co-E), individually and collectively referred to herein as "propylene polymer" for convenience "PP", which, upon exposure to atmospheric pollutants, is resistant to "gas fade". Gas fade is evidenced by objectionable color. More particularly, the invention relates to PP fiber essentially free of a hindered phenol anti-oxidant and pigmented with a white pigment or with a lightly colored pigment. By "essentially free" I refer to the fact that the only antioxidant which may be present is that added to the polymerization reactor in which PP is polymerized, to facilitate the drying and processing of the PP. This invention was born of the need for protecting the effect of the pigment in PP fiber made from general purpose PP.
Recent advances in PP fiber technology have been instrumental in propagating use of the fiber for fabric used in a wide variety of applications in many of which the conditions of exposure of the fabric are conducive to gas fade (of the fabric). Such fabric is used in the automobile industry for upholstery, headliners, trim and carpets. The fabric for trim is typically non-woven. Much of the interior of an automobile uses needle-punched fabric, and the major portion includes woven, knit or tufted structures. Needle-punched non-wovens offer economic advantages and when combined with pigmented PP fibers, may provide good resistance to fading of the color under the elevated temperature conditions typically encountered in an automobile and other applications where such PP fibers are used.
The problem is that PP is degraded under the action of oxygen and light, particularly ultraviolet (uv) light, even when thermal degradation is not a consideration. Currently, commercially available unpigmented, uv-stabilized PP fiber develops an objectionable yellow color upon exposure to atmospheric pollutants including nitrogen oxides. This development of yellow color affects the color provided by the pigment. Upon exposure to light, PP fiber also suffers a severe loss of physical integrity over time, the higher the intensity of light, the worse the degradation. For light stabilization, conventionally used stabilizers are of the hindered amine, benzophenone, benzotriazole, and cyanoacrylate types, inter alia. Oxidative degradation is conventionally combatted by the addition of antioxidants to the PP. Among these are the hindered phenols, sulfur-containing and phosphite type antioxidants.
Despite such stabilizing efforts, PP fabric made with pigmented PP fiber stabilized with an antioxidant, is particularly susceptible to yellowing caused by nitrogen oxides in waste gases, especially those from industrial plants, automobiles and burning of oil in oil stoves. The yellowing is easily noticeable upon storage of PP fabric at room temperature, the longer the storage period, the worse the yellowing. We recently realized that it was because of the necessary stabilization against degradation by oxygen, heat and light, that the yellowing of PP fibers, exposed to atmospheric pollutants, persisted. We have now resolved this problem so that yellowing is minimal, by the choice of a particular type of hindered amine uv-stabilizer, provided essentially no antioxidant is present in the PP fiber.
Though considerable effort was devoted, in the recent past, to the study of the details of the mechanisms and kinetics of oxidative and light degradation, and the yellowing of PP fibers, particularly those stabilized with hindered phenol type antioxidants, it was never suspected that the presence of a hindered phenol moiety, typically present in an antioxidant, contributed to the yellowing of the fibers. Of course, the problem was exacerbated when the PP fibers of the fabric were pigmented with white (Titanox) pigment, or a pale pigment in which the light color is commonly contributed by any one of a wide array of dyes. Pastel colors, and azo dyes which provide the colors, are highly favored for their cosmetic appeal.
We have now found that the resistance of pigmented PP fiber to the peculiar yellowing due to nitrogen oxides, known as "gas fade" is unexpectedly improved by the use of a certain type of hindered amine stabilizer provided that a phenol-containing antioxidant be absent, or be maintained at a very low level solely to assist in processing the resin, the level preferably being less than about 0.05 parts per hundred parts (phr), or less than 500 parts per million (ppm) of PP. Additives other than an antioxidant may be added to facilitate processing, if the color they contribute is desirable, and they do not degrade the physical properties of the PP. Such additives may include lubricants in addition to alkaline earth metal stearates, near-colorless or white fillers such as glass fibers or talc, and optical brighteners.
European Patent No. GB 0,197,793 discloses that PP was stabilized against gas fade by incorporating a specific phenolic type antioxidant (AO) containing a tetraoxaspiro undecane structure, in the PP resin. But we have found any antioxidant to be contraindicated for improving resistance of pigmented PP fiber to gas fade.
We have now found further evidence that hindered amines with a specific structure, which amines exhibit excellent uv stabilization in numerous host polymers without notably distinguishing one polymer from another as far as their relative susceptibility to uv stabilization is concerned, are also surprisingly effective to stabilize PP against gas fade. Moreover, the evidence is that these amines are effective when used without an AO of any kind, except for the trace amounts, less than 100 ppm, which might be used in the manufacture of the PP resin, to improve its stability during processing. Since some of these amines are better uv stabilizers than other such amines, but make the PP fibers more susceptible to yellowing, it is most preferred to use a mixture of the amines for optimum results.
The peculiarly distinguishing structural feature of such effective hindered amines having such a beneficial effect on resistance to gas fade, is that they contain, as an essential portion of their basic structure, a polysubstituted (hereafter also referred to as "substituted" for brevity) piperazin-2-one (PSP) having an N.sup.1 -adjacent carbonyl, and at least the C.sup.3 atom (carbon atom in the 3-position in the ring) has two substituents (hence "polysubstituted"), which may be cyclizable, that is, form a cyclic substituent.
Though such compounds, referred to in U.S. patents listed herebelow, were known to be excellent UV stabilizers in colorless organic materials when used in combination with AOs, there was nothing to suggest that their incorporation in PP, and in nylon and polyester fibers to a lesser extent, among a wide array of other polymers, in the absence of an AO, and preferably also in the absence of a hindered phenol, phosphite or thioester stabilizer, would provide effective stabilization against gas fade.